Unless you’ve, kind of, stumbled into  a certain kind of disaster movie, seeing a comet in the sky is a wonderful treat.

I mean that tail? Spectacular.

You’re not gonna find those anywhere else. Or are you? Well to human eyes, from Earth’s vantage point, the tail of a great comet can’t be beat.

But comets are far from the only  cosmic bodies that can grow a tail. Here are five other cosmic wonders  that leave a little bit of themselves behind as they careen through space. [♪ INTRO] In the grand scheme of the universe,  comets and asteroids are pretty similar. I mean, asteroids are generally more rocky, and comets are more icy, but  they’re both small clumps of stuff left over from our solar system  forming a bunch of planets.

But comets are known for growing tails  when they get too close to the Sun, and asteroids are not. When our Sun heats up a  comet, a bunch of ice and dust get vaporized into a sort of  temporary atmosphere called a coma. That coma is then blasted off  the space rock by solar radiation to create a vibrant and reflective light show.

But there’s nothing stopping  an asteroid from having stuff that the Sun can blast away, too. So several asteroids in our solar system  are known to have comet-like traits. And one classic example is Phaethon,  which has shed so much material over the eons that its debris cloud is thought to  be the source of the Geminid meteor shower.

Usually, making a meteor shower  debris cloud is a comet’s job. But you try telling Phaeton what to do. Astronomers have known about  Phaethon’s tail since 2009.

And for a long time, they  assumed it was made out of dust. Which makes sense, right? Comets have tails full of dust, and  Phaethon likes to cosplay as a comet.

But upon closer inspection, Phaethon’s tail was way too  robust to be made of just dust. So, there had to be something else going on. And eventually, astronomers managed to figure out that the tail was mostly made of sodium vapor.

See, Phaethon doesn’t just get  a bit too close to the Sun. It gets closer to the Sun than Mercury, and far closer than any other  notable asteroid ever does. And because it gets so close, it gets  so hot that sodium inside the asteroid gets turned into a vapor, and eventually leaks up through the rest of the rock until it  escapes and gets shaped into a tail.

Since comets can also release sodium vapor, you’d think we would have figured out this  was happening to Phaethon a while ago. But no, the paper that actually proved  it was happening was published in 2023. And although the paper answered  one question, it raised others.

Like, if it’s not trailing a bunch  of dust and pebbles and such, how did Phaethon make the debris cloud  that the Geminids are coming from? The team did have a possible answer to that: Maybe there was some ancient collision  between Phaethon and another body. But what that body was, and when that collision happened  is still just a bunch of shrug emojis.

And Phaethon isn’t the only solar system  body rocking a sodium-powered tail. As I mentioned before, comets  can shoot out sodium vapor. But our next entry is a tag team  of two much, much larger objects.

They’re so large their gravity has  smushed them each into spheres. Dead, dark grey spheres. Of course I’m talking about Mercury and the Moon.

Mercury’s sodium tail was discovered back in 2001. And just like Phaethon, it's  produced by the Sun’s heat. But it does have its own mystery.

Because Mercury’s tail isn’t at its  brightest when it’s closest to the Sun. It’s actually brightest exactly  16 Earth days after that. And astronomers don’t know why.

Meanwhile, our moon is much further away,  but that doesn’t stop it from shedding a bunch of sodium that the Sun’s  radiation shapes into a long tail, too! And since the Moon orbits the Earth, there are a few days each lunar month… a few days around each New Moon… where our planet actually  passes through that tail. During this time, the Earth’s gravity  even causes some of that sodium to narrow in on a particular part of the sky.

And because sodium glows a  particular shade of yellow, this means we see a greater concentration  of yellow light in one patch of sky, about five times bigger  than the Moon appears to be. Astronomers call it the Sodium Moon Spot. But before you get your hopes up, its light is way too dim for you  to see it with the naked eye.

But it’s worth mentioning that the Moon’s  tail isn’t just made out of sodium. Nor is Mercury’s. Sodium is just better at making its presence  known, in the form of that yellow light.

And that makes it a good way  to study the tail as a whole. For example, scientists want to understand  how Mercury’s barely-there atmosphere changes as the Sun gets more or  less active throughout the year. And as far as our lunar companion  goes, studying the Sodium Moon Spot can help astronomers understand what’s  creating the Moon’s tail at all.

Because according to research published in 2021, the spot doesn’t seem to get  brighter when the Sun is more active. It does get brighter when the lunar surface is getting pelted by more micrometeoroids, though. So maybe its origin story is a  bit different from Mercury’s, despite the two bodies looking  so similar at first glance.

But lest you think tails are an  Inner Solar System phenomenon, let’s turn our attention to Pluto. Because if any rock in the solar  system is going to have a tail, it had better be the one that  shares its name with a cartoon dog. When the New Horizons spacecraft flew by in 2015, astronomers got an up-close-if-not-personal  look at Pluto’s tail.

And despite the dwarf planet being  basically five billion kilometers from the center of the solar system,  our Sun is still up to its old tricks. Its radiation is definitely a lot  more spread out at those distances, but there’s still enough to smash  into Pluto’s thin atmosphere and push molecules like nitrogen  and methane into a tail. And on top of that, the collisions  between the Sun’s particles and Pluto’s can knock electrons  from one particle over to another.

It’s a process called charge  exchange, and those electrons wind up having so much excess energy that they have to emit x-rays in order to calm down. I mean, as much as a subatomic particle  can experience a sense of calm. It’s these x-rays that researchers used to  discover Pluto’s tail in the first place.

But they started hunting for it before  New Horizons made its closest approach. See, astronomers already knew  that comets give off x-rays for the exact same reason. So in 2014, they used the Chandra X-ray  Observatory to see if Pluto had them, too.

And after staring straight  at Pluto for almost 10 hours, Chandra captured a whopping two x-rays. Which sounds like nothing. But it’s not.

It’s two more than nothing. And two whole x-rays can be a significant  amount to an x-ray astronomer. But by combining that Chandra  research with New Horizons’ data, scientists stumbled into a bit of a mystery:  Pluto’s tail seems to be too bright.

Because while it’s shedding  enough atmosphere into its tail, the Sun’s influence shouldn’t be strong  enough to create that many x-rays. This could just mean that  Pluto’s tail is actually bigger than New Horizons measured it to be. Or maybe there are some weird magnetic goings on in the region that are funneling  the Sun’s radiation toward Pluto.

We’ll have to wait for a more sensitive  x-ray telescope to take a look. Which…could be a while, so let’s move on to a tail that is way more impressive than anything  a dwarf planet could hope to wag. This is WASP-69b, or at least an  artist’s rendition of WASP-69b.

It’s roughly 160 light years away. And it orbits so close to its star, one of  its years lasts less than four of our days. Given what we’ve learned about our Sun  and bodies like Phaethon and Mercury, it should come as no surprise that WASP-69b’s star is slowly stripping off the exoplanet’s  atmosphere and turning it into a tail.

But WASP-69b is a gas giant  that’s larger than Jupiter, and about as massive as Saturn. So unlike Phaethon and Mercury, this tail is big. And it was only recently that  astronomers learned just how big it was.

According to a paper published in 2024, this  tail is at least 580,000 kilometers long, which is more than three times  longer than the planet is wide. And to figure that out, astronomers  used the Keck observatory in Hawaii to hunt not for sodium, or  for the occasional x-ray, but for a helium signature buried in the infrared. Which is actually a novel technique  for exoplanet tail hunting.

Past research had focused on an  ultraviolet hydrogen signal, instead. But that signal isn’t visible  from below Earth’s atmosphere, which Keck very much is. Plus, when you use a space-based telescope,  you have to worry about your data getting muddied by a bunch of hydrogen  that’s floating around in space but not a part of the tail you’re trying to study.

So switching to helium, when you’ve got a planet that’s mostly made of hydrogen and  helium, seems like a good way to go. However you’re looking at this particular tail, it’s clear a lot of matter is being blasted off. About one Earth’s worth every billion years.

But given that WASP-69b is about 90 Earth masses, no one has to worry about  it completely blowing away. At least not anytime soon. And because astronomers have managed  to find another rare exoplanet that’s actively being stripped of its atmosphere, they can watch a strange form of  planetary evolution happen in real time.

In fact, WASP-69b and its super  long tail may help provide answers to a few outstanding mysteries. For example, most of the worlds  that we’ve spotted super close to their stars are either small rocky  worlds or Jupiter-sized gas giants. There’s an apparent dearth  of so-called Hot Neptunes, and astronomers aren’t exactly sure why.

But however long it takes to answer that question, we can safely assume that WASP-69b will  still be around to congratulate us. Now so far, we’ve been talking  about stars creating tails by blowing a bunch of stuff off  of other, much smaller objects. But it turns out, stars aren’t  just the engines behind tails.

They can grow them, too! Or, at least, one can. Meet Mira, a binary star system that’s  home to a dying red giant called Mira A, and its dead white dwarf companion Mira B.

And as they traverse interstellar space  together, Mira A is shedding a bunch of mass and making a real pretty tail that  clocks in at about 13 light years long. For comparison, that’s about  three times the distance between our solar system and the next one over. The tail formed because the Mira  binary is traveling a bit faster through space than all the stuff around  it, called the interstellar medium.

Well, it's not just a bit faster. Mira is more like the stellar  version of supersonic jet, creating a bow shock and wake  as it plows through space. And remember how Pluto’s tail creates x-rays?

Particles from different sources  colliding with each other and all that? Well, Mira’s situation isn’t completely different. Basically, you’ve got a bunch of super  hot electrons coming out the bow shock mixing with the cooler hydrogen molecules  in the wake that Mira is shedding.

Collisions between the two excites  a bunch of those hydrogen molecules, which have to glow in order to  relax back to their regular state. But unfortunately, this glow  isn’t visible to the human eye.  Mira’s tail shines in the ultraviolet  part of the electromagnetic spectrum. But luckily for us, we have  telescopes that can see UV light.

And the one that captured it is called GALEX. GALEX is an all-sky surveyor, which  means it scans huge swaths of sky and astronomers peruse the resulting  images to see if anything cool pops up. And Mira’s tail is one of those cool things!

Back in 2006, a team just happened  to be looking at some incoming data, and found a smudgy patch of  light in Mira’s vicinity. Something that looked more like  a nebula than anything else. So they looked closer.

And voila! Other astronomers followed up on  that accidental discovery, too. And a year after the first paper  about Mira’s tail was published, there was one about what it  looks like to a radio telescope.

So now that we know Mira’s tail is  there, it can help teach us about what might happen to other  stars that aren’t massive enough to end their lives in a violent supernova. And yes, that includes the Sun. But luckily for us, we’ll be  long gone by the time our star balloons into a red giant and  maybe grows a tail of its own.

To any aliens out there though, looking our way, I hope they see something spectacular. And you know what else is spectacular? The limited edition pin we're selling this month.

You can purchase your very own tiny  version of Pluto, tail included, by heading over to dftba.com/SciShow And thanks for watching. [♪ OUTRO]